PSN-L Email List Message

Subject: Re: Newbie questions...
From: John Hernlund hernlund@.......
Date: Tue, 06 Jun 2000 13:37:50 -0700 (MST)

On Wed, 7 Jun 2000, Steve Olney wrote:
> 1. All the seismograms I have seen on the sites show the P wave as being
> smaller than the S wave.  As I understand it the P wave is longitudinal and
> the S wave is traverse, i.e. they are orthogonal.
> Question: Wouldn't the relative amplitudes then depend on the orientation of
> a horizontal seismograph (i.e. a Lehman)?   I think I can understand that a
> vertical seismograph would show the higher S wave close to an event, but the
> situation for a horizontal one has me puzzled.

Absolutely.  Same with the P waves too.  I have a horizontal seismograph
oriented to swing NW-SE in Tempe, AZ and last week there was a quake down near
Yuma, almost directly SW of here.  So the seismogram I recorded had almost no
P wave, since the P component would have been moving NE-SW relative to my
station, and had a large shear component which was moving NW-SE at my
location.  If I had a seismometer which was oriented at right angles so that
it swung NE-SW then that would have picked up a good P and a weak S.  With all
three components of motion I could have used this information, along with the
P and S arrival times, to try and estimate the exact location of the quake
using only one station.  Unfortunately this is not always so accurate and many
stations used at once greatly improve the estimate.

   I'll leave the rest of your questions to other members of the group who can
probably provide better answers...

Good luck!

John Hernlund
E-mail: hernlund@.......



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Larry Cochrane <cochrane@..............>